Browsing by Author "Sankar, Raman"

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Epitaxial growth of vertically stacked p-MoS2/n-MoS2 heterostructures by chemical vapor deposition for light emitting devices
(2017-02-01)
Nikam, Revannath Dnyandeo
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Sonawane, Poonam Ashok
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Sankar, Raman
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Chen, Yit-Tsong
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應用化學系
Despite the dramatic advances of two-dimensional vertical heterostructure, the controlled growth and potential applications in light emitting devices of these heterostructures have not yet been well established. Here, we report, for the first time, the epitaxial growth of two-dimensional p-MoS2/n-MoS2 vertical heterostructures by a chemical vapor deposition (CVD) method, where the n-MoS2 was synthesized first, followed by an epitaxial growth of p-MoS2 on top of the n-MoS2 via a control of the growth temperature. Atomic-resolution scanning transmission electron microscopy (STEM) imaging reveals that the vertically stacked bilayer of the hexagramshaped p-MoS2/n-MoS2 preferred the 2H stacking phase during the growth. The structural and optical features of the as-grown p-MoS2/n-MoS2 heterostructure were examined by Raman and photoluminescence (PL) spectroscopy. This novel hybrid heterostructure was demonstrated to be an excellent building block for a highly efficient white light emitting diode (WLED). In addition, we transferred the p-MoS2/n-MoS2 on top of a p-GaN bilayer to fabricate a tetra-layered (4-L) n-MoS2/p-MoS2/p-GaN heterostructure, which could emit electroluminescence (EL) in forward bias. The EL spectra comprise three emission peaks centered at 481 nm (from pGaN), 525 nm (from p-MoS2), and 642 nm (from n-MoS2), with a dominant emission peak located at 642 nm. The WLED device composed of the n-MoS2/p-MoS2/p-GaN heterostructure showed a luminance of 30,548 cd/m(2), luminescence efficiency of 29% and the luminous efficacy of 294 lm/W at a bias voltage of 4 V. This work demonstrates that white light emission can be generated from vertically stacked few-layered 2D materials-based heterostructures, which also hold great potential for constructing color-tunable light emitters for low-cost display, lighting, and optical communication.
Femtosecond time-evolution of mid-infrared spectral line shapes of Dirac fermions in topological insulators
(2020-06-17)
Yeh, Tien-Tien
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Tu, Chien-Ming
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Lin, Wen-Hao
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Cheng, Cheng-Maw
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Tzeng, Wen-Yen
Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (Delta R/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the Delta R/R to show their intrinsic ultrafast dynamics.
Intrinsic Carrier Transport of Phase-Pure Homologous 2D Organolead Halide Hybrid Perovskite Single Crystals
(2018-12-27)
Li, Min-Ken
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Chen, Tzu-Pei
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Lin, Yen-Fu
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Raghavan, Chinnambedu Murugesan
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Chen, Wei-Liang
This work reveals the intrinsic carrier transport behavior of 2D organolead halide perovskites based on phase-pure homologous (n = 1, 2, and 3) Ruddelsden-Popper perovskite (RPP) (BA)(2)(MA)(n-1)PbnI3n+1 single crystals. The 2D perovskite field effect transistors with high-quality exfoliated 2D perovskite bulk crystals are fabricated, and characteristic output and transfer curves are measured from individual single-crystal flakes with various n values under different temperatures. Unipolar n-type transport dominated the electrical properties of all these 2D RPP single crystals. The transport behavior of the 2D organolead halide hybrid perovskites exhibits a strong dependence on the n value and the mobility substantially increases as the ratio of the number of inorganic perovskite slabs per organic spacer increases. By extracting the effect of contact resistances, the corrected mobility values for n = 1, 2, and 3 are 2 x 10(-3), 8.3 x 10(-2), and 1.25 cm(2) V-1 s(-1) at 77 K, respectively. Furthermore, by combining temperature-dependent electrical transport and optical measurements, it is found that the origin of the carrier mobility dependence on the phase transition for 2D organolead halide perovskites is very different from that of their 3D counterparts. Our findings offer insight into fundamental carrier transport behavior of 2D organic-inorganic hybrid perovskites based on phase-pure homologous single crystals.
Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators
(2015-09-15)
Tu, Chien-Ming
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Yeh, Tien-Tien
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Tzeng, Wen-Yen
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Chen, Yi-Ru
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Chen, Hsueh-Ju
Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located similar to 1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu0.02Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.
Selenium nanoparticle prepared by femtosecond laser-induced plasma shock wave
(2020-01-06)
Tzeng, Wen-Yen
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Tseng, Ya-Hsin
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Yeh, Tien-Tien
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Tu, Chien-Ming
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Sankar, Raman
A novel approach for the production of both amorphous and crystalline selenium nanoparticles (SeNPs) using femtosecond laser-induced plasma shock wave on the surface of Bi2Se3 topological insulators at room temperature and ambient pressure is demonstrated. The shape and size of SeNPs can be reliably controlled via the kinetic energy obtained from laser pulses, so these are applicable as active components in nanoscale applications. Importantly, the rapid, low-cost and eco-friendly synthesis strategy developed in this study could also be extendable to other systems. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
THz Generation and Detection on Dirac Fermions in Topological Insulators
(2013-11-01)
Luo, Chih Wei
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Chen, Hsueh-Ju
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Tu, Chien Ming
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Lee, Chia Ching
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Ku, Shin An
Ultralow Schottky Barriers in Hexagonal Boron Nitride-Encapsulated Monolayer WSe2 Tunnel Field-Effect Transistors
(2020-04-22)
Pande, Gaurav
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Siao, Jyun-Yan
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Chen, Wei-Liang
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Lee, Chien-Ju
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Sankar, Raman
To explore the potential of field-effect transistors (FETs) based on monolayers (MLs) of the two-dimensional semiconducting channel (SC) for spintronics, the two most important issues are to ensure the formation of variable low-resistive tunnel ferromagnetic contacts (FCs) and to preserve intrinsic properties of the SC during fabrication. Large Schottky barriers lead to the formation of high resistive contacts, and methods adopted to control the barriers often alter the intrinsic properties of the SC. This work aims at addressing both issues in fully encapsulated ML WSe2 FETs using bilayer hexagonal boron nitride (h-BN) as a tunnel barrier at the FC/SC interface. We investigate the electrical transport in ML WSe2 FETs with the current-in-plane geometry that yields hole mobilities of similar to 38.3 cm(2) V-1 s(-1) at 240 K and on/off ratios of the order of 10(7), limited by the contact regions. We have achieved an ultralow effective Schottky barrier (similar to 5.34 meV) with an encapsulated tunneling device as opposed to a nonencapsulated device in which the barrier heights are considerably higher. These observations provide an insight into the electrical behavior of the FC/h-BN/SC/h-BN heterostructures, and such control over the barrier heights opens up the possibilities for WSe2-based spintronic devices.